[R] nlme correlated random effects

[Daniel O'Shea]

I am examining the following nlme model.

asymporig<-function(x,th1,th2)th1*(1-exp(-exp(th2)*x))
mod1<-nlme(fa20~(ah*habdiv+ad*log(d)+ads*ds+ads2*ds2+at*trout)+asymporig(da.p,th1,th2),
    fixed=ah+ad+ads+ads2+at+th1+th2~1,
    random=th1+th2~1,
    start=c(ah=.9124,ad=.9252,ads=.5,ads2=-.1,at=-1,th1=2.842,th2=-6.917),
    data=pca1.grouped)

However, the two random effects (th1 and th2) which describe the asymptotic relationship between richness (fa20) and area (da.p) are correlated: -0.904 with approximate 95% ci of -0.99 to -.32.
I examined the anova of mod1 with both random effects and mod2 with just th1 and mod1 is preferred.  I also examined pdDiag(th1 + th2~1) for another model (mod3) and based on the anova the original mod1 is preferred.

My question is can I use pdBlocked with only 2 random effects or should I and if so how I would specify that in the model or perhaps the 95% ci for correlation is wide enough to ignore???

Dan